Variable inactivation of human factor VIII from different sources by human factor VIII inhibitors

von Willebrand factor in factor VIII concentrates protects against neutralization by factor VIII antibodies of haemophilia A patients

We investigated the neutralization activity of factor VIII (FVIII) antibodies of 12 haemophilia A patients, acquired during treatment with plasma-derived FVIII concentrates. All plasma samples, drawn in a clinically stable situation before any immunotolerance treatment, contained anti-A2 domain and anti-light-chain FVIII antibodies. In nine patients' plasmas, containing relatively high amounts of FVIII light-chain antibodies (53-96%), a higher neutralization activity was found against recombinant FVIII concentrate (Recombinate) than against plasma-derived von Willebrand factor (vWF)-containing concentrate (Haemoctin SDH). No difference in neutralization of the two concentrates was found in two patients' plasmas with almost equal content of FVIII light- and heavy-chain antibodies, or one plasma with predominantly heavy-chain antibodies. These results suggest that haemophilia A patients with relatively high amounts of FVIII light-chain antibodies in plasma might benefit by infusion of FVIII concentrates containing vWF because vWF appears to have some protective effect on FVIII. This hypothesis should be tested by a clinical study.

Variation in inhibitor reactivity in acquired haemophilia A with different concentrates

Acquired haemophilia A due to the development of auto-antibodies directed against factor VIII (FVIII) is a rare disorder that may result in serious haemorrhagic episodes. Although in many cases no associated underlying disorders are apparent, the condition has been reported in association with autoimmune disorders, haematological malignancies, treatment with certain drugs and pregnancy. The reaction kinetics of auto-antibodies to FVIII differ from those observed with allo-antibodies in congenital haemophilia. Previous studies in congenital haemophilia have raised the possibility that inhibitory antibodies vary in their reactivity with the factor VIII molecules in different concentrates used for treatment. However, the interaction of FVIII in concentrates and inhibitors in acquired haemophilia has never been previously studied. In this study, the effect of different FVIII concentrates was studied on neutralization in vitro by performing inhibitor titres using the New Oxford inhibitor assay method. The inhibitor titre in eight patients with acquired haemophilia A was assayed against five commercially available FVIII concentrates of varying purity. The intermediate purity concentrate 8Y and the high purity concentrate that contains normal amounts of von Willebrand's Factor (vWF) (Alphanate) gave lower titres than the high purity concentrates with low (Monoclate-P) or no (Kogenate) von Willebrand content. All but one antibody had very low reactivity with porcine FVIII. Further work will be required to establish whether concentrates manifesting a low level of in vitro reactivity with the inhibitor have a better haemostatic effect in vivo.

Mechanism and Kinetics of Factor VIII Inactivation: Study With an IgG4 Monoclonal Antibody Derived From a Hemophilia A Patient With Inhibitor

The development of an immune response towards factor VIII (fVIII) remains a major complication for hemophilia A patients receiving fVIII infusions. The design of a specific therapy to restore unresponsiveness to fVIII has been hampered by the diversity of the anti-fVIII antibody. Molecular analysis of the specific immune response is therefore required. To this end, we have characterized an fVIII-specific human IgG4κ monoclonal antibody (BO2C11) produced by a cell line derived from the memory B-cell repertoire of a hemophilia A patient with inhibitor. BO2C11 recognizes the C2 domain of fVIII and inhibits its binding to both von Willebrand factor (vWF) and phospholipids. It completely inhibits the procoagulant activity of native and activated fVIII, with a specific activity of approximately 7,000 Bethesda units/mg. vWF reduces the rate of fVIII inactivation by BO2C11. The antibody-fVIII association rate constant (kass ∼7.4 × 105M−1 s−1) is eightfold lower than that for vWF-fVIII association, whereas its dissociation rate constant (kdiss ≤1 × 10−5s−1) is 100-fold lower than that for the vWF-fVIII complex, which suggests that BO2C11 almost irreversibly neutralizes fVIII after its dissociation from vWF. BO2C11 is the first human monoclonal anti-fVIII IgG antibody that has been isolated and allows the study of fVIII inactivation at the molecular level.

Mechanism and Kinetics of Factor VIII Inactivation: Study With an IgG4 Monoclonal Antibody Derived From a Hemophilia A Patient With Inhibitor

The development of an immune response towards factor VIII (fVIII) remains a major complication for hemophilia A patients receiving fVIII infusions. The design of a specific therapy to restore unresponsiveness to fVIII has been hampered by the diversity of the anti-fVIII antibody. Molecular analysis of the specific immune response is therefore required. To this end, we have characterized an fVIII-specific human IgG4κ monoclonal antibody (BO2C11) produced by a cell line derived from the memory B-cell repertoire of a hemophilia A patient with inhibitor. BO2C11 recognizes the C2 domain of fVIII and inhibits its binding to both von Willebrand factor (vWF) and phospholipids. It completely inhibits the procoagulant activity of native and activated fVIII, with a specific activity of approximately 7,000 Bethesda units/mg. vWF reduces the rate of fVIII inactivation by BO2C11. The antibody-fVIII association rate constant (kass ∼7.4 × 105M−1 s−1) is eightfold lower than that for vWF-fVIII association, whereas its dissociation rate constant (kdiss ≤1 × 10−5s−1) is 100-fold lower than that for the vWF-fVIII complex, which suggests that BO2C11 almost irreversibly neutralizes fVIII after its dissociation from vWF. BO2C11 is the first human monoclonal anti-fVIII IgG antibody that has been isolated and allows the study of fVIII inactivation at the molecular level.

Acquired inhibitors

Factor VIII auto-antibody inhibitors, though rare, may present significant and often life-threatening haemorrhage. These auto-antibodies, arising predominantly in older individuals, occur in association with autoimmune disorders, lymphoproliferative disorders, solid tumours, medications and the postpartum state. Almost half of the patients develop auto-antibodies spontaneously without an underlying medical condition. Factor VIII auto-antibody inhibitors are characterized as polyclonal IgG immunoglobulins directed against the FVIII procoagulant activity. Laboratory diagnosis is made by performing the aPTT clotting time in conjunction with a mixing study, and subsequently with specific factor assays. Auto-antibodies are quantified most commonly utilizing the Bethesda assay. Acquired inhibitors to other coagulation factors, including factors IX, XI, XIII, vWF protein, and the vitamin K-dependent proteins are extremely rare. The principles of therapy are similar to those which apply to the management of factor VIII auto-antibodies. Treatment of patients with acquired factor VIII auto-antibody inhibitors varies depending upon the underlying medical condition, the titre of the inhibitor, and the clinical presentation. Acutely bleeding patients with high-titre auto-antibodies generally respond well with infusions of porcine factor VIII concentrate, PCCs or rFVIIa. Extracorporeal plasmapheresis with exchange will acutely reduce circulating antibodies and can be used in conjunction with factor infusions and/or IgIV. Haemorrhage in a patient with a low titre auto-antibody will usually respond to high doses of human factor VIII concentrate. DDAVP may also increase factor VIII levels in patients with low-titre inhibitors. Long-term reduction of auto-antibodies can be achieved by immuno-suppressive regimens using steroids and/or cytotoxic agents, IgIV and interferon-alpha. The selection of the appropriate treatment depends upon the associated medical condition, likelihood of spontaneous remission, risk of toxicities of therapy and cost. Determining the efficacy and safety of new treatment modalities for factor VIII auto-antibodies and other coagulation factor inhibitors will require multicentre randomized clinical trials.

Autoantibody to factor VIII that has less reactivity to factor VIII/von Willebrand factor complex

To determine the difference in reactivity of factor VIII (FVIII) inhibitor to FVIII/von Willebrand Factor (vWF) complex and FVIII free of vWF, an autoantibody to FVIII light chain was tested. A patient (1-3) suffered from autoimmune hemolytic anemia with autoantibody to FVIII. Epitope specificity of the patient's IgG (I-3 IgG) was shown to be the C2 domain of FVIII light chain (2170-2332) by Western blotting using recombinant FVIII deletions expressed in Escherichia coli. The inhibitory effect on FVIII procoagulant activity (VIII:C) of I-3 IgG was tested against a conventional FVIII concentrate; Haemate P, a monoclonal antibody-purified FVIII concentrate; Hemofil M, and a recombinant FVIII (rFVIII); Kogenate. I-3 IgG showed only 1.3 BU/mgIgG for Haemate P, in contrast to 20 BU/mgIgG for both Hemofil M and Kogenate. The ratio of VIII:C/vWF:Ag in Haemate P and Hemofil M was 1/3.43 and 1/0.01, respectively, while Kogenate did not contain vWF. The inhibitory effect of the I-3 IgG was then compared with Kogenate and its complex with vWF. The inhibitory effect was decreased against the rFVIII by forming a complex with vWF from 22 BU/mgIgG to 0.5 BU/mgIgG. Fab from the I-3 IgG had the same effect. In addition, vWF showed a protective effect on FVIII inactivation by the I-3 IgG in a dose dependent manner. Fifty-nine percent of residual VIII:C was retained in the presence of 8 U/ml of vWF after 1 hr incubation with I-3 IgG. These results suggested that vWF could compete with the I-3 IgG for binding to FVIII.